Pipelines are frequently protected from corrosion below ground using coal tar or asphalt waterproofing coating, as well as microcrystalline wax. Generally, the coat tar or asphalt pitch contain talc or other fillers and are then referred to in the industry as coal tar or asphalt enamels. The application temperature of the hot applied coating material ususally varies between 350.degree. to 500.degree. F., depending upon the composition of the tar or asphalt enamel, ambient temperature, and other variables. A solution primer compatible with the hot applied coating is generally applied ahead of the waterproofing material to improve the adhesion or bond of the coating to the pipe. Hot applied asphalt, or coal tar wax, or wax coatings in the ground not externally mechanically shielded from the backfill will become displaced and distorted owing to soil pressure. Weight of the backfill tends to cold flow the coating on the pipe and displace or remove it from the pipe surface exposing bare metal to the soil. Various mechanical shields are used to minimize this problem. Generally, a 15 pound per square (100 ft.sub.2) asbestos felt similar to roofing felt is spirally wrapped as a shielding over hot applied coal tar or asphalt coating to prevent soil stress. Also, polyethylene film has been used as a outer wrap.
Fiberglass mats have been used to reinforce the tar as an inner wrap. They do not level the tar evenly, as they are relatively thick (normally 20 mils) and will not saturate well at the spiral laps. Since they have low tensile strength and are made of very short fibers, the glass mats are reinforced about every 1/2" with glass threads running parallel to the edge of the wrap. Another disadvantage of the glass mats is that the binders are bulky. For example, a glass mat with a small amount of glass in ounces per hundred square feet contains more binder than glass. This results in bulky rolls that are unwieldy to apply and require a great number of rolls or stops and starts.
A major problem inherent in the application of the hot applied materials is the variation in thickness built up due to the solidification of the hot applied liquid materials on the pipe in irregular surface contours. These varying thicknesses are the result of the mechanical application means used to flow the enamel onto the pipe. It is desirable to obtain a uniform thickness, both from standpoint of material cost and from the standpoint of design for adequate waterproofing protection. Such hot applied coatings of coal tar or ashalt are tyically applied in a thickness of about 3/32", with minimum thicknesses recommended in industry practices of approximately 1/16".
Pipelines are coated by mill application of the coating or by line-travel application. In mill application, the coating is applied soon after the pipe is made. It is later moved to the right-of-way from the coating location for installation with the coating on the pipe when it arrives at the job site. It is necessary to leave about six inches uncoated at each end of the pipe to prevent the heat from the welding operation burning and damaging the coating. This is referred to as a cut-back area on the mill coating. It does require a subsequent field applied coating to be made on each weld, which is approximately every 40 ft. Another disadvantage of mill coating is that there are typically about 30 bends to the mile, and considerable coating repair has to be made if the bends are severe owing the damange of the coating from the field bending. Another method of coating which is practical in some instances, as in rural areas, is to coat the pipe after it is welded. In this method of coating, the pipe is coated and immediately lowered to the ditch ready for back-fill. This eliminates coating the field joints, and there are no bend repairs, as the coating is applied to the pipe after it is bent. This method of coating requires considerable and large equipment on the right-of-way and is impractical to use in congested areas. The coating and wrapping method selected is determined by the location of the pipeline in regard to which coating application procedure is the most practical for the general construction problem.
In either method, the hot enamel as it flows on to the pipe is irregular in its surface and thickness. The material applies like a heavy coat of paint, with runs and sags. The pipe heat sinks and solidifies the enamel. The solidified coating results in a surface having considerable variations in thickness, due to the runs and sags. In the mill application, the pipe is spirally rotated as it is coated and wrapped, and centrifugal force causes ridges and high and low spots in the waterproofing coating when it is in the liquid phase. These tend to solidify into relatively thick and thin areas in the coating. Line travel application of coating also has the disadvantage that the coating on the lower half of the pipe tends to drip and fall off the pipe surface due to its weight. The hot coating needs to be held to the surface and leveled immediately after application before it solidifies and before the mechanical shielding wrap is applied for soil-stress resistance.
Glass fiber mats made of short fibers two or three inches long held together with various binders have been used as an inner wrap for hot coating. They have not been effective with respect to leveling the enamel, because the mats are of irregular surface density which permits the enamel to bleed through certain parts of surface faster than others. Also they have, generally speaking, about every half inch across the width of the mat a glass thread running parallel to the edge of the wrap to give it tensile strength. The binder in the glass mats has no tensile strength at the enamel application temperature, so the glass threads are required. These glass threads have been known to wick water where the coating is damaged, which frequently happens in the lowering-in operations. The wicking of water has caused corrosion, especially where the glass threads might have pulled down through the hot enamel because of improper tension control to the pipe surface. Glass mats have not proved to be dependable as far as achieving a good leveling effect on the hot applied coating, owing to the low tensile strength and varying surface permeability of the glass, as well as its short fiber length.